I want to build a good game, like most down here. In order to build a good game you need a decent budget to buy assets and hire people. Now, is it legal to use patented things like music from Muse for example as background music in my game? I won't be asking money for the game, I don't care about making any money unless I really need to in order to build quality games. So the idea would be, taking a genre think of an idea, program it and use already made quality assets from everywhere I can find something that can suit the game.
Whether you're charging for your game or not is irrelevant; you can't use anyone else's work without permission. There are free assets out there, but you have to make sure they are legitimately free according to the license.
You might want to take a look at Creative Commons, and their licenses. There exist a wealth of music (and I think there's artwork aswell) licensed under creative commons that you may use for your game (IANAL). Take a look in google and see what you can find.